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Evasiveness of acyclicity of undirected graph
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evasivenessacyclicityundirectedgraph
Problem
The lecture note by Jeff Erickson discusses "Evasive Graph Properties":
We call a graph property evasive if we have to look at all $\binom{n}{2}$ entries in the adjacent matrix to decide whether an (undirected) graph has that property.
As an example, it shows that connectivity is evasive.
I want to know: Is acyclicity evasive? How to prove/disprove it?
My attempt: A rather straightforward adaptation I made from the proof for connectivity does not work for acyclicity.
We call a graph property evasive if we have to look at all $\binom{n}{2}$ entries in the adjacent matrix to decide whether an (undirected) graph has that property.
As an example, it shows that connectivity is evasive.
I want to know: Is acyclicity evasive? How to prove/disprove it?
My attempt: A rather straightforward adaptation I made from the proof for connectivity does not work for acyclicity.
Solution
Consider any algorithm for acyclicity. We will use the following adversary:
Whenever the algorithm asks for an edge, say that the edge exists if it connects two different connected components. (That is, if it doesn't close a cycle.)
You can show that the graph constructed by this dialog is always a forest, and moreover the constructed graph has the same connected components as the query graph (which contains all queried edges). In particular, when the last edge is asked, the graph is a tree (assuming $n \geq 3$; otherwise every graph is acyclic), and the graph is acyclic if and only if this edge does not belong to the graph.
Whenever the algorithm asks for an edge, say that the edge exists if it connects two different connected components. (That is, if it doesn't close a cycle.)
You can show that the graph constructed by this dialog is always a forest, and moreover the constructed graph has the same connected components as the query graph (which contains all queried edges). In particular, when the last edge is asked, the graph is a tree (assuming $n \geq 3$; otherwise every graph is acyclic), and the graph is acyclic if and only if this edge does not belong to the graph.
Context
StackExchange Computer Science Q#75683, answer score: 3
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